A group III nitride, for example, GaN (gallium nitride) based semiconductor is expected as a material for a next-generation power semiconductor device. The GaN based semiconductor device has a wide band gap compared to Si (silicon), and can realize a high withstand voltage and a low loss compared to a Si semiconductor device.
A GaN based transistor is generally applied with a high electron mobility transistor (HEMT) structure in which a two-dimensional electronic gas (2DEG) is used as a carrier. A general HEMT is a normally-on transistor which comes to be a conduction state even when a voltage is not applied to a gate. There is a problem in that it is difficult to realize a normally-off transistor which does not come to be the conduction state as long as the voltage is not applied to the gate.
In power circuits handling large power such as several hundreds of voltage to a thousand voltage, a normally-off operation is required for the safety. Therefore, there is proposed a circuit configuration in which the normally-on GaN based transistor and the normally-off Si transistor are connected in cascode to realize the normally-off operation.
However, in such a circuit configuration, a gate voltage of the normally-on GaN based transistor cannot be made sufficiently high. Therefore, there is a problem in that an on-state current does not flow sufficiently.